Cathode Side of a PEM Fuel Cell

In low-temperature cells, oxygen required for the ORR is transported to the CCL through the gas channels and the GDL. Understanding the potential losses resulting from oxygen transport in the cell components is one of the key issues for better cell design. 

The EIS has been widely used to study this problem. In general, FIS can give separate contributions of each transport and kinetic process to the total cell resistance. However, unambiguous interpretations of EIS spectra require modeling. 

A widely used approach toward the interpretation of cell impedance is a method of equivalent circuit (Orazem and Tribollet, 2008). The great advantage of the method is simplicity. However, as discussed in the section Introductory Remarks, the equivalent circuit is not unique, that is, similar spectra could be generated by different circuits. In recent years, there has been a growing interest in the direct physical modeling of the cell impedance. 

Springer et al. (1996) developed a physical model for the impedance of the cathode side of a PEFC, taking into account oxygen transport in the GDL. They fitted the 

A physical-based impedance model of the PEFC cathode, which included oxygen transport in the GDL, has been developed by Bultel et al. (2005). The authors reported qualitative similarity of measured and calculated impedance spectra and analyzed the effect of GDL diffusion resistivity on the cell performance. 

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Figure 3.2 The high-current polarization curve of the cathode side of a PEM fuel cell, Eq. (3.7). Note the limiting current density at j = jo-...

Jiao, K., B. Zhou, and P. Quan. 2006a. Liquid water transport in parallel serpentine channels with manifolds on cathode side of a PEM fuel cell stack. Journal of Power Sources 154 124-137. 

The results of the section Impedance of the Cathode Side of a PEM Fuel Cell, have been obtained assuming constant oxygen stoichiometry X of the flow. However, impedance experiments are usually performed at a constant oxygen flow rate, rather than at a constant X. Indeed, keeping constant X means that the inlet flow rate must be changed in phase with the mean current density perturbation, which is hardly possible. 

The main components of a PEM fuel cell are the flow channels, gas diffusion layers, catalyst layers, and the electrolyte membrane. The respective electrodes are attached on opposing sides of the electrolyte membrane. Both electrodes are covered with diffusion layers, and the flow channels/current collectors. The flow channels collect current from the electrodes while providing the fuel or oxidant with access to the electrodes. The gas diffusion layer allows gases to diffuse to the electro-catalysts and provides electrical contact throughout the catalyst layers. Within the anode catalyst layer, the fuel (typically H2) is oxidized to produce electrons and protons. The electrons travel through an external circuit to produce electricity, while the protons pass through the proton conducting electrolyte membrane. Within the cathode catalyst layer, the electrons and protons recombine with the oxidant (usually 02) to produce water. 

Conventional reference electrodes consist of a solid reversible electrode and an aqueous electrolyte solution. To measure the individual contributions from the anode and the cathode of a PEM fuel cell, the electrolyte solution of the reference electrode must either be in direct contact with one side of the solid proton exchange membrane or be located in a separate compartment with electrical contact between the reference electrode and the solid membrane by means of a salt bridge [66], As a result, two different types of reference electrode configurations are employed for the study of fuel cells internal and external. 

Fig.l (a) Principal layout of a PEM fuel cell with the main functional components, viz. proton-conducting polymer-electrolyte membrane (PEM), catalyst layers on anode (ACL) and cathode sides (CCL), gas-diffusion layers (CDL) and flow fields (FF). (b) Disciplines in fuel cell research and how they are connected by the theory. 

Dutta et al. [54] used the unified approach to study mass transport between the channels of a PEM fuel cell with a serpentine flow field. Their model is three-dimensional and allows for multi-species transport. They studied the effect of flow channel width in the serpentine flow field on velocity distribution, gas mixture distribution and reactant consumption. Serpentine flow fields allow for a greater area for diffusion of the supply gases. Their results showed that for low humidity conditions, water transport is dominated by electro-osmotic effects, i.e., water flows from anode to cathode at the side of the cell closer to the gas channel inlet. At the outlet side of the cell, water transport is dominated by back diffusion, and it flows in the opposite direction. Thus the serpentine flow field allows for circulation of the water within the cell. 

The conversion of chemical energy to electrical energy in a PEM fuel cell occurs through a direct electrochemical reaction. It takes place silently without combustion. The key part of a PEM fuel cell, which is known as a membrane electrode assembly (MEA), consists of a polymer electrolyte in contact with an anode and a cathode on either side. To limction, die membrane must conduct hydrogen ions (protons) and separate either gas to pass to the other side of the cell. A schematic representation of a PEM fuel cell is shown in Figure 1.5. 

Qualitatively, this picture is analogous to carbon corrosion on the cathode side in the hydrogen-depleted region of a PEM fuel cell discussed in the section Carbon Corrosion in PEFCs from Hydrogen Depletion. The major difference is that in PEFC, hydrogen depletion causes reversal of the depleted domain due to the presence of oxygen on the anode side. In DMFC, no reversal happens, the anode in the MD-domain continues generating protons, but the source of these protons is CCR. 

In a PEM fuel cell, a membrane having both anode and cathode sides coated with CLs is called an MEA. The MEA is the core of a PEM fuel cell, as it provides the location for the electrochemical reactions. A typical five-layer MEA includes the anode GDL, anode CL, membrane, cathode CL, and cathode GDL. As the reaction in a CL is three phased, an MEA should provide passages for the transport of electrons, protons, and reactant gases. Furthermore, it should also have the ability to remove product water. During the recent decades of fuel cell development, many MEA stmctures have been developed, and they will be discussed in detail in Chapter 2. 

Transparent fuel cells [77-84] are widely used to characterize the water removal process on the flow field of a PEM fuel cell. As they allow optical access to the flow field, one can observe the formation of water droplets and the water removal process in the flow channels. A transparent cell usually includes a transparent plastic end plate, a copper plate to serve as the current collector, and a flow field plate on the anode side, cathode side, or both, as shown in Fig. 3.22. For the test setup, a high-speed camera is required to record the status of liquid water in the flow channels. Thus, water flooding at different stages or under various conditions can be recorded, as shown in Fig. 3.23 [81 ]. In this way, the visualization of water transport and removal in a transparent cell can be used to optimize the operating conditions, the structural designs of the flow field and gas diffusion layer, and the screening of materials for the gas diffusion layer. 

Another method to determine current distribution in a PEM fuel cell was presented by Sun et al.169 in which they designed a current distribution measurement gasket that can be placed anywhere in the fuel cell (usually at the back of the cathode side) and can measure the local current density at various point along the active area of the cell. The advantage of this approach is that it can be used without having to modify any component of the cell. The same technique was also used by Zhang et al.170 to compare the performance of interdigitated and serpentine flow fields. 

It is mainly the PEM that distinguishes a PEM fuei ceii from aii other types of fuel cells. As its name implies, a PEM has the capability of transporting protons. It is typicaiiy made of a solid ionomer with acidic groups such as sulfonic acid (-SO3H) at the end of the polymer side chains. Polystyrene sulfonic acid is one such ionomer, and it was used as the PEM in the early days of the PEM fuel cell development around the 1960s. However, since the PEM fuel cell environment is warm, corrosive, and oxidative (at cathode), an ionomer with higher chemical and electrochemical stability is required. State-of-the-art PEMs are made of perfluorinated polysulfonic acids, and include DuPont s Nafion . 

The heart of the PEM fuel cell is the proton exchange membrane, which transports protons from the anode to the cathode. The membrane also serves to separate the fuel and oxidant gas phases and electronically insulates the cathode from the anode. The most typical membrane is a sulfonated per-fluorinated polymer. The Nation family of membranes made by DuPont is representative of this class, and is based on a sulfonated tetrafluoroethylene-based fluoropolymer-copolymer, with the chemical structure represented in Figure 1.2. The sulfonic acid (SO3H) groups on the side chains allow the protons or other cations to "hop" from one acid site to another, in the presence of water. The exact mechanism of the proton movement is an area of significant research. An active area of research is the development of hydrocarbon-based... 

The end plate is another important component in a PEM fuel cell or fuel cell stack. End plates are placed at each end of the fuel cell anode and cathode sides. Sometimes, the end plates also serve as flow field plates with flow channels on one side. For example, when a metal plate is used as the flow field plate, the end plates at the anode and cathode sides also serve as the flow field plates on their respective sides. However, a separate plate is often used as the end plate to assemble a fuel cell if graphite plates are used as the flow fields. To reduce the weight of fuel cell stacks, robust but light materials, such as aluminum and polymers, are often used for the end plates. 

The ionic conductivity of PEM is significantly dependent on the membrane hydration. Inadequate membrane hydration results in high electrical resistance as well as the formation of dry and hot spots leading to membrane failure. The electroosmotic transport occurs due to the proton transport. Proton migrations drag water along with it from the anode side to the cathode side that can eventually reduce the membrane hydration and block the active reaction site in the CCL. Water transport process in a PEM fuel cell is a complex phenomenon, hence it is essential to make a delicate water balance for better and optimum fuel cell performance, and prevent material degradation (Das et al., 2010). 

Cho et al. [140] examined the performance of PEM fuel cells fabricated using different catalyst loadings (20, 40, and 60 wt% on a carbon support). The best performance—742 mA/cm at a cell voltage of 0.6 V— was achieved using 40 wt% Pt/C in both anode and cathode. Antonie et al. [28] studied the effect of catalyst gradients on CL performances using both experimental and modeling approaches. Optimal catalyst utilization could also be achieved when a preferential location of Pt nanoparticles was close to the PEM side ... 

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